1. Field of the Invention
This invention relates to ink jet recording and more particularly, to recording papers for the ink jet recording.
2. Description of the Prior Art
Great interest has recently been attracted to the recording by ink jet systems because of their reduced noise, ease in color recording, possibility of high speed recording, and utilization or ordinary papers. The ink jet systems are now being used widely in the field of facsimile, various types of printers and the like. It is generally accepted that ordinary papers are satisfactorily usable as recording paper for use in the ink jet recording system. However, this does not mean that all the ordinary papers which are widely used at present are usable. In order to obtain recorded matters of more excellent quality, the recording paper itself should meet several requirements which follow: (1) The paper must have excellent ink receptivity to allow ink dots deposited on the paper surface to be rapidly absorbed in the inside of paper; and (2) The paper must prevent ink dots applied on the surface from running or spreading.
The requirement (1) is the most fundamental one which must be furnished with ink jet recording papers and assumes great importance especially when color images are produced by the ink jet system. This is because in order to produce color images, it is necessary to make a variety of colors from combinations of yellow, cyan and magenta inks, so that inks of different colors are deposited on the same portion of the paper surface, resulting in large amounts of inks per unit area.
The requirement (2) is necessary for obtaining clear recorded matters. By preventing ink dots from spreading, the optical density of recorded matter can be increased. In general, the simplest method of increasing the optical density of recorded matter is to increase the concentration of dye in the ink. However, this method has its limit because of the tendency to clog a head nozzle. Accordingly, it is important that recording papers satisfy the above requirement.
Aside from the fundamental requirements (1) and (2), recording papers should satisfy the following further requirements: (3) The degree of penetration of ink in the direction of depth or in the longitudinal direction is not too great; and (4) The paper has an excellent brightness. The optical density of recorded matter largely depends on the state of the paper surface and if the degree of the penetration in the direction of depth is too great, it is difficult to make the optical density high.
The recording paper to be applied in the ink jet recording system is generally made from bleached chemical pulp to which fillers, dyes and, if required, sizing agents and strength improvers are added.
There have heretofore been proposed several types of papers for ink jet recording. For instance, Japanese Laid-open Patent Application No. 52-74340 discloses an ink jet recording paper which is characterized in that a ratio of an air resistance to basis weight (g/m.sup.2) (air resistance/basis weight) is below 0.3 and that when an aqueous ink for ink jet recording is dropped in an amount of 0.004 ml, an absorption time of ink is in the range of from two seconds to 60 seconds. Further, Japanese Laid-open Patent Application No. 52-53012 teaches a method of making recording papers which is characterized by applying a coating to a base paper which has been incorporated with a wet strength improver known per se and which has a Stockigt sizing degree of below 1 second whereby the resulting surface coated paper has a Stockigt sizing degree of below 3 seconds. In these laid-open patent applications, there are described surface sizing agents including oxidized starch, PVA, galactomannon gum, polyacrylamide, sodium alginate, styrene-maleic acid copolymer, CMC and other cellulose derivatives, casein, soy bean protein and the like. In addition, there are mentioned, as sizing additives, hydrophobic materials or latices, rosin and its derivatives, petroleum resins, fumaric acid, maleic acid its derivatives, waxes, synthetic resins, fatty acids, alkylketene dimers and the like, and, as pigment or filler, kaolin, calcium carbonate, aluminium hydroxide, satin white, titanium oxide, and urea-formalin organic fillers.
Moreover, there is proposed in Japanese Laid-open Patent Application No. 55-5830 a sheet for ink jet recording which comprises a support and an ink-receptive layer formed on the surface of the support, said sheet having an opacity of 55.0 to 97.5%, an absorptivity of the ink-receptive layer being in the range of 1.5 to 18.0 mm/min. Also, Japanese Laid-open Patent Application No. 55-11829 teaches a sheet for ink jet recording which has (1) two or more layers, (2) an opacity of 55.0 to 97.5%, (3) a top layer with a thickness of 1.0 to 16.0 microns, and (4) an ink-receptivity of the top layer of 1.5 to 5.5 mm/min and that of a second layer of 5.5 to 60.0 mm/min.
The ink-receptive layer of these sheets is formed of white pigments such as clay, talc, diatomaceous earth, calcium carbonate, calcium sulfate, barium sulfate, titanium oxide, zinc oxide, zinc sulfide, satin white, aluminium silicate, lithopone and the like. As binder resin, there are mentioned oxidized starch, etherified starch, gelatin, casein, carboxymethyl cellulose, hydroethyethyl cellulose, polyvinyl alcohol and SBR latex.
We have made an extensive studies of ink jet recording papers which satisfy the afore-mentioned requirements (1) to (4) and found that coating layers made of combinations of sizing agents or binder resins and pigments and fillers which have been known from the prior laid-open patent applications do not show satisfactory characteristics when applied for such recording purpose. Especially, the resins serving as the sizing agent or binder play an important role and it has been found that the known resins are unsatisfactory in ink receptivity and thus recorded matter using such resins as a surface coating frequently shows insufficient optical density.